The present disclosure generally relates to control systems, and more particularly to control systems for controlling the power and heating/drying rate of microwave dryers.
Conventional heating or drying typically comprising convection, or a combination of convection and radiative gas or electric resistance heating, was commonly used in the manufacturing of ceramic materials. However, the slow heating rate and poor temperature control associated with these conventional heating methods results in a high energy consumption and inconsistent product quality. Furthermore, utilization of these two modes of heating typically result in thermal differences within the ceramic body, due to the fact that these two heating modes are applied only to the surface and rely on thermal conductivity of the ceramic body to effect the temperature beneath the surface to the center of the piece.
Industrial heating by microwave radiation has been successfully used to accelerate the drying of traditional ceramics. In comparison with convection heating, microwave heating provides a higher heating rate, where there is sufficient absorption, with better temperature control, and thus results in lower energy consumption and potentially better quality products. Furthermore, the utilization of microwave energy can deliver a uniform application of the energy to the ceramic article, rather than to the article surface, as is the case for the aforementioned convection and radiative modes of heating. Lastly, microwave heating is much faster than conventional drying.
Although microwave heating is faster and more efficient than conventional modes such as convection and radiative heating, standard microwave heating typically involves controlling the amount of microwave energy utilizing a constant power setpoint to determine the amount of microwave energy to apply to the ceramic body. Typically, this power output is set at some value that ensures that the reflected power never exceeds the manufacturer's specification; i.e., a power output assuming a constant load and material dielectric characteristics. This conventional method of controlling microwave heating does not account for variations in the amount of mass of material in the microwave dryer (loading), or variations in the dielectric characteristics of the load, or variations in geometries and densities of the load. As a result, the microwave heating can be inefficient because the power input at various times during heating is not properly adjusted.
Microwave drying is a drying process that can be employed in ceramic filter and substrate production lines. In ceramic filter production lines, ceramic logs can be passed through dryers and applicators that use microwave energy to dry the ceramic logs or wares. If the drying of the logs is not uniform, then the logs can have defects such as grooves, cracks, end flares, hot logs or cold logs, etc. Prior to each applicator, at the end of each dryer, and/or at the end of the drying process, the temperatures of the logs can be measured using a pyrometer to determine the extent to which the ceramic logs have been dried. Logs that are too hot after the dryers can release organics prior to the firing process which may be detrimental to the final log quality. Logs that are too cold after the dryers, may still contain wet regions that prevent further processing, particularly through the subsequent cutting process, and may also be detrimental to the final log quality.
While the abovementioned techniques have proven useful, developing improved fabrication and control techniques with improvement in performance over existing technology is desirable.